An introductory Guide on electrical fuses

in English

This is a guide to help you understand the workings of fuses, the various types of fuses available in the market, the different standards of fuses, and what you should look for when buying a fuse. This guide is meant to help hobbyists and practicing engineers choose the correct fuse for their applications.

What is an Electrical Fuse?

An electric fuse is a device included in a circuit to protect your wiring and circuit components. It safeguards your devices by limiting the excessive current flowing through them during an overload or fault conditions like a short circuit or ground. A fuse comprises a fusible link (or links) encased in a tube and connected to a contact terminal. The electrical resistance of these fusible links is so low that it (normally) acts as a conductor. These fuses should be connected in series with each phase conductor to allow the current to flow through it. For every fuse, the manufacturer mentions the rating (usually in amperes), which decides the maximum amount of current the fuse will carry without opening. When the circuit current flow exceeds this value, the fuse link melts due to the heat generated by the current flow and opens the circuit.

Figure 1: An electrical fuse (Image courtesy: LITTELFUSE)

Meaning of the term ‘Speed of Fuse’

The speed of the fuse is the time it takes to rupture. It depends on the fuse element material, the overload current, and the ambient temperature. Here are the types of fuses based on the speed of blow:

Very fast acting fuses (Flink Flink/FF): These fuses primarily provide short circuit protection in consumer electronics devices (PCs, TVs, etc.), Industrial controllers (PLCs, DCS, etc.), Automotive control units (ECU, TCU, etc.). The fuse element of these fuses is made up of silver and opens within the first half cycle of overcurrent.
Fast-blow (Quick-acting) fuses (Flink /F): They are designed to protect general-purpose electrical circuits and cabling from overloads and short circuits. The fuse element of fast blow fuses is made with copper and has a tolerance limit of 500% of rated current for 0.05-2 sec.
Slow-acting fuses (Trage/T): They are designed to tolerate short-duration current surges. They are commonly used in applications with unexpected momentary overloads, such as motor and lighting circuits. The fuse element of slow-acting fuses is also made with copper and has a tolerance limit of 500% of rated current for a minimum of 10 sec.
Medium blow fuses (Mitteltrage/M): They are used in automotive, commercial, and industrial electronics to protect the circuits subjected to moderate to high inrush current. The fuse element of fast blow fuses is made with copper and has a tolerance limit of 500% of rated current for 4-6 sec.

How many types of fuses are available in the market today:

There are three main categories of fuses available in the market based on different requirements, which are as follows:

DC fuses: DC or direct current fuses are designed explicitly for use in DC circuits. They are larger than AC fuses and reduce the arc by increasing the distance between the electrodes.
AC fuses: In an AC system, the magnitude and polarity of current and voltage vary concerning time. This variation in voltage helps terminate the arc produced by the heat generated in the fuse element due to excess current. Therefore, AC fuses are slightly smaller compared to DC fuses.
Cartridge fuses: Cartridge fuses are very popular in most industrial and commercial applications because they have a wider range of types, sizes, and ratings. The fuse element of these fuses is enclosed in a glass or ceramic tube and secured to end caps that are firmly attached to the body of the fuse. These fuses are easy to replace and can be rated at voltages up to 1500V and have an interrupting rating from 600mA to 200kA.
Figure 2: Cartridge fuses (Image courtesy: SCHURTER)
HRC fuses: Also known as high-rupturing capacity fuses, HRC cartridge fuses are used in HV systems up to 11kV and are available in ranges up to 350A. They protect HV systems, including transformers, capacitor banks, cables, and overhead power lines, against short circuits.
Figure 3: HRC fuses (Image courtesy: Eaton Bussmann)
Automotive fuses: These protect various electrical and electronic components, such as headlamps, wipers, horns, and fuel pumps. In today's vehicles, the most common fuse used are blade fuses. Fuses with ceramic bodies and bolt-in terminal designs are commonly used. However, some older vehicles still use cartridge fuses. Blade fuses are available in three types: The standard blade, the minifuse, and the maxi-fuse. The minifuse is the most commonly used circuit protection device. Minifuses are available in ratings from 5 to 30 amps. The maxi-fuse is a serviceable replacement for a fusible link cable. It is used in circuits that have high operating currents. Maxifuses are available in 2–100 amp ratings; the most common is the 30 amp. Bolt in fuses is used in hybrid, electric, or conventional vehicles for power distribution and has high current ratings from 80- 350 amps.
Figure 4: Automotive fuses (Image courtesy: Littelfuse)
Alarm-indicating fuses: Alarm-indicating fuses feature an integral mechanism that activates an alarm contact when the fuse blows. They are designed to reduce downtime by immediately pinpointing the blown circuit. Alarm-indicating fuses are used in machine tools, computers, control circuits, and telecommunications equipment.
Figure 5: Alarm Indicating fuses (Image courtesy: Eaton)
Industrial & electrical power fuses: These fuses have high interrupting ratings and protect systems such as power transformers, distribution transformers, instrument transformers, and Photovoltaic applications, where circuit breakers may not protect the system. They are rated up to 1.5kV.
Figure 6: Industrial & electrical power fuses (Image courtesy: Siemens)
Thermal fuses: Thermal fuses, also known as Thermal cutoff (TCO), are similar to regular fuses, except they open an electrical circuit when it exceeds a specific temperature rather than a certain magnitude of current. Two main types of thermal fuses are available: One-Time Use and Resettable. One-time use fuses permanently cut off the circuit once triggered by heat and need replacement. Common applications include electrical appliances such as coffee machines, hand dryers, motors, cookers, etc. Resettable thermal fuses can automatically reset after the temperature cools down. They are less common but may be used in specific applications where resetting is beneficial.
Figure 7: Thermal fuses (Image courtesy: Bourns)
PCB fuses: They prevent damage to sensitive electronic components in printed circuit boards (PCBs) from short circuits, inrush currents, and voltage transients. These are used in battery chargers, consumer electronics, power supplies, industrial controllers, etc.. PCB fuses are available with axial lead and radial lead options.
Figure 8: PCB fuses (Image courtesy: BEL Fuse)
Surface-Mount Device (SMD) fuses are designed for Surface-Mount Technology (SMT) applications. These fuses are mounted directly onto the PCBs and safeguard sensitive components from overcurrent conditions. Their compact size and surface-mount design make them ideal for space-constrained applications like smartphones, tablets, LED Lighting, and automotive control units.
Figure 9: SMD fuses (Image courtesy: Panasonic)
Semiconductor fuses: Semiconductor fuses: They are very fast-acting and are commonly referred to as I2T fuses. They protect power semiconductors like diodes, SCRs, GTOs and IGBTs from the effects of a short circuit. They have super-quick disconnect characteristic, faster than that of conventional low voltage HRC fuses. They are used in DC links, power converters, UPS systems, and soft starters for motors.
Figure 10: Semiconductor fuses (Image courtesy: SIBA)
Polymeric Positive Temperature Coefficient (PPTC) fuses: They provide a resettable overcurrent protection alternative, reducing warranty, service, and repair costs. PPTC fuses are suitable for applications where frequent overcurrent conditions occur, or constant uptime, such as consumer electronics, power lines, telecom, automotives, I/O ports, process control, and medical equipment protection applications.
Figure 11: Semiconductor fuses (Image courtesy: Littelfuse)

When Choosing a Fuse, What is Important?

Selecting the correct fuse for a particular application involves many factors, some of them are:

Current Rating: It is defined as the maximum current that can flow through the fuse element continuously without exceeding the permissible temperatures. When choosing a fuse, it is recommended that the current rating be as close as possible to the system's standard running current.

Voltage Rating: It is the maximum allowable working voltage of the fuse under safe working conditions. The voltage rating of the fuse must be equal to, or greater than, the available circuit voltage. Exceeding the voltage ratings or using an AC-only fuse in a DC circuit may lead to severe fuse failure.

Ambient temperature: The current carrying capacity test of fuses is carried out at 25°C and depends on ambient temperature. Higher ambient temperature reduces current capacity and shortens the fuse’s service life. Therefore, working at a comparatively low temperature can extend the service life of a fuse.

Voltage drop: It refers to the voltage drop generated by a fuse when rated current flows through it, reflecting the capacity of the internal resistance of the fuse. A fuse's voltage drop should be as minimal as possible to reduce power losses.

Breaking capacity: The breaking capacity is the value of the prospective breaking current that a fuse element can withstand at a specified voltage.

I² t ratings: Also known as the melting integral of a fuse. This parameter relates with the energy utilized by the fuse to clear the Electrical Overcurrent Current (EOC) event. I2t is the thermal energy required to melt a specific fuse element and is determined by the element’s construction, materials, and cross-sectional area.

Frequently Asked Questions (FAQs)

What is a fuse holder? It is necessary to use.

A fuse holder is a casing used to insert the fuse into electrical and electronic circuits. Although it is not necessary to use a fuse holder every time, having one simplifies the installation of the fuse. It also becomes easier to remove the fuse and isolate the circuit to carry out maintenance work.

What is an eFuse, and in which applications are they used?

eFuses, or electronic fuses, are ICs designed to replace conventional fuses or other protection devices such as PPTC fuses. They are housed in small plastic packages and include a control circuit and a low-on-resistance power switch that connects the input port to the load. eFuse offers much faster and more precise protection and does not require replacement after actuation. These fuses are used in servers, HDD/SSD, network switches, industrial controllers, and home appliances.

What are the different global standards for fuses?

Fuses are subject to various standards depending on region and application. UL 248, CSA22 and NEC standards are mainly used in the United States. IEC 60127 is the safety standard for fuses destined for international use. Also, there are some application-specific standards like ISO 8820, SAE J554, and JASO D612 are used for automotive, IEC 60269-4 for semiconductor Protection, UL 248-19, and IEC 60269-6 for photovoltaic systems.

What are class CC fuses?

Class CC fuses are designed according to North American standards UL 248-4 and CSA 22.2 for branch circuit protection. These cylindrical fuses measure 13/32’’ x 1 1/2’’ and feature a high-rated breaking capacity of 200 kA with 600 V AC, with rated currents of up to 30 A. Class CC fuses are ideal for protecting relatively small branch circuits in high-power feeds. These fuse links have a small-diameter rejection tip on the contact cap, making them visually distinct from cylindrical fuses.

Where can I buy fuses and associated accessories at the best prices?

Farnell Electronics offers a comprehensive line of fuses and associated accessories for automotive, electronic, and industrial applications at the best price from leading suppliers. The product line includes fuse assortments, fuse holders, fuse drawers, fuse clips, fuse design kits, and fuse pullers, enabling easy fuse installation and replacement.